There needs no 'lack' in a language to motivate borrowing, as is evident by the fact that Japanese continues to use numerals originally borrowed from Middle Chinese (ichi, ni, san) side-by-side with its older, native set (hitotsu, futatsu, mittsu).

If that's correct, that makes sense to me because Indic short vowels would then become Old/Middle Chinese short vowels... so then to respond to your question, there is nothing abnormal about the vowels at all, only in our modern understanding of the phonetics of Old and Middle Chinese.

This changing in the number of tones from 8 to 4 is just as innovative as from 8 to 10; it is not "un-innovating" or purifying the language of innovations either, as the 4 tones of Standard Mandarin are not the 4 tones of Middle Chinese.